Hydrothermal carbonization of algal feedstocks and fuels produced thereby

ABSTRACT

In one embodiment, the present disclosure provides a method for producing a solid fuel. A feedstock that includes algae or delipidized algal residue and a liquid carrier is heated to a suitable temperature, at a suitable pressure, and for a suitable amount of time to form a desired amount of solid hydrochar. The hydrochar is collected and compressed into a compressed solid.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of, and incorporates by reference, U.S. Provisional Patent Application Ser. No. 62/011,793, filed Jun. 13, 2014.

This disclosure relates generally to the hydrothermal carbonization of feedstocks that include algae or an algal component, and fuels produced therefrom. In particular embodiments, the hydrochar produced from the hydrothermal carbonization reaction is compressed, such as being formed into pellets.

SUMMARY

In one embodiment, the present disclosure provides a method for producing a solid fuel from a feedstock that includes algae or delipidized algal residence and a liquid carrier. In particular implementations, the feedstock include algae, delipidized algal residue, or a mixture of algae and delipidized algal residue. The feedstock optionally can include additional components, such as other types of biomass. In some aspects of the present disclosure, the feedstock includes less than about 20% by weight of algae or delipidized algal residue, such as between about 2% and about 10% by weight of algae or delipidized algal residue.

The feedstock is heated at a suitable temperature and pressure, and for a suitable period of time, to form solid hydrochar. The temperature, in particular implementations, is between about 120° C. and about 250° C., between about 150° C. and about 250° C., or between about 180° C. and about 250° C. The pressure is between about 2 bar and about 40 bar, such as between about 5 bar and about 40 bar, in a particular example. In a particular implementation, the reaction is carried out for a period of between about 5 minutes and about 16 hours. In various examples, the reaction is carried about for between about 5 minutes and about 2 hours, such as between about 5 minutes and about 1 hour.

In a specific implementation, after reaction, the solid hydrochar is collected and compressed into a compressed solid. For example, the compressed solid may be pellets or briquettes. In some implementations, the solid hydrochar is compressed, or capable of being compressed, without the addition of an external binder. In some implementations where an external binder is used, the binder is less than about 10% by weight of the material to be compressed, such as less than about 5% by weight. In additional aspects of the present disclosure, the hydrochar formed from the method is combined with hydrochar produced from a lignocellulosic feedstock and then compressed to a solid.

In another implementation, the hydrochar is subjected to an extrusion process. In one example, the hydrochar is formed and then subjected to an extrusion process. In another example, the hydrothermal carbonization reaction is carried out as part of the extrusion process. For example, the extrusion process may generate sufficient heat and pressure to carry out the hydrothermal carbonization reaction.

In yet another implementation, the hydrochar is collected but not compressed, or is not specifically collected. For example, the hydrochar may be subjected to another reaction or process.

In some aspects of the disclosed method, the hydrochar is compressed at a temperature of between about 25° C. and about 200° C., such as between about 100° C. and about 160° C., between about 120° C. and about 180° C., between about 140° C. and 160° C., or less than about 200° C. In another aspect of the disclosed method, the hydrochar is compressed at a temperature less than a temperature typically used to compress hydrochar produced from the hydrothermal carbonization of lignocellulosic biomass.

In another embodiment, the present disclosure provides a method for producing a solid fuel using a feedstock that includes delipidized algal residue and a liquid carrier. In particular examples, the feedstock consists of, or consists essentially of, delipidized algal residue and the liquid carrier. The liquid carrier, in particular implementations, is water.

In particular aspects of the present disclosure, the feedstock used in the method includes a biomass component, and the delipidized algal residue comprises at least about 25% of the biomass component, such as at least about 50% or at least about 75%. The feedstock optionally can include additional components, such as other types of biomass. In implementations of the method where an additional biomass component is used, in a particular example, the additional biomass includes algae.

The feedstock is heated at a suitable temperature and pressure, and for a suitable period of time, to form solid hydrochar. The temperature, in particular implementations, is between about 120° C. and about 250° C., between about 150° C. and about 250° C., or between about 180° C. and about 250° C. The pressure is between about 2 bar and about 40 bar, such as between about 5 bar and about 40 bar, in a particular example. In a particular implementation, the reaction is carried out for a period of between about 5 minutes and about 16 hours. In various examples, the reaction is carried about for between about 5 minutes and about 2 hours, such as between about 5 minutes and about 1 hour.

After reaction, the hydrochar is typically collected. Optionally, the hydrochar may be compressed into a compressed solid. For example, the compressed solid may be pellets or briquettes. In some implementations, the solid hydrochar is compressed, or capable of being compressed, without the addition of an external binder. In some implementations where an external binder is used, the binder is less than about 10% by weight of the hydrochar material to be compressed, such as less than about 5% by weight. In additional aspects of the present disclosure, the hydrochar formed from the method is combined with hydrochar produced from a lignocellulosic feedstock and then compressed to a solid.

In some aspects of the disclosed method, the hydrochar is compressed at a temperature of between about 25° C. and about 200° C., such as between about 100° C. and about 160° C., between about 120° C. and about 180° C., between about 140° C. and 160° C., or less than about 200° C. In another aspect of the disclosed method, the hydrochar is compressed at a temperature less than a temperature typically used to compress hydrochar produced from the hydrothermal carbonization of lignocellulosic biomass.

In some cases, the hydrochar is not specifically collected. For example, the hydrochar may be subjected to another reaction or process.

In a particular implementation, the hydrochar is subjected to an extrusion process. In one example, the hydrochar is formed and then extruded. In another example, the extrusion process is part of the hydrothermal carbonization process. An extrusion process may, for example, generate the desired heat and pressure conditions for a hydrothermal carbonization reaction.

The disclosed method can provide greater energy densification between the feedstock and the hydrochar than comparable hydrothermal carbonization of lignocellulosic feedstocks. In one particular example, when the hydrothermal carbonization reaction is carried about between about 120° C. and about 215° C., such as between about 120° C. and about 200° C. or between about 120° C. and about 175° C., the energy densification may be at least about 1.1

In another aspect, the disclosed method can allow for lower temperatures and pressures to be used than in hydrothermal carbonization of lignocellulosic feedstocks. In another aspect, the energy content of the hydrochar is significantly higher than the energy content of hydrochar produced from lignocellulosic biomass under the same reaction conditions. In yet another aspect, the method produces hydrochar having an energy content that is at least about equivalent to the energy content of lignocellulosic hydrochar produced at a reaction temperature that is at least about 30° C. higher, such as at least about 50° C. higher or at least about 60° C. higher, than that used to produce the hydrochar from the feedstock including biomass from an algal source, such as whole algae or delipidized algae.

In another aspect, the present disclosure provides a solid fuel comprising hydrochar formed from the hydrothermal carbonization of algae, delipidized algal residue, or a mixture thereof. The solid fuel may be, for example, pellets or briquettes. In another example, the solid fuel is an extruded material.

In one implementation, the solid fuel comprises at least about 5% of biomass of algal origin treated via hydrothermal carbonization, or “algal hydrochar,” according to a method of the present disclosure. For example, the solid fuel may be at least about 10%, about 25%, about 50%, about 75%, about 85%, about 90, or about 95% of algal origin/algal hydrochar. In one example, being of algal origin means being algae, delipidized algal residue, or a mixture thereof. In a more specific example, being of algal origin means being whole (non-delipidized) algae. In another specific example, being of algal origin means being delipidized algal residue.

In some examples, the solid fuel, such as briquettes or pellets, including solid fuels from the previously described implementation, include less than about 10% by weight of an external binder, such as less than about 5%, 4%, 2%, or 1%. In another example, the solid fuel does not include an external binder.

In another implementation, the present disclosure provides solid fuels from hydrothermal carbonization of algal sources that have higher energy densities than solid fuels produced from hydrothermal carbonization of lignocellulosic sources. For example, the solid fuel may have an energy density of 1.1 or higher.

Some aspects of the present disclosure provide solid fuels, such as compressed solids or extruded materials, that demonstrate improved stability. In one example, the fuel, after being immersed in water for 60 minutes, exhibits a stability of at least about 55%, such as at least about 60%, about 65%, about 70%, about 75%, about 80%, about 90%, or about 95%. In a specific example, stability is defined as the pellet weight after tumbling divided by the pellet weight before tumbling, multiplied by one hundred. For example, the stability may be the pellet of an immersed pellet after tumbling, divided by the weight of the immersed pellet before tumbling, multiplied by one hundred. In a yet more specific example, the immersion test is the immersion test described herein.

In some implementations, the fuels with enhanced stability include less than about 10% by weight of an external binder, such as less than about 5, about 4%, about 2%, or about 1%. In a specific example, the solid fuel does not include an external binder, or is substantially free of external binders. In further implementations, including examples with the previously described limits on external binders, the fuel includes at least about 10%, about 25%, about 50%, about 75%, about 85%, about 90, or about 95% hydrochar of algal origin, such as algae or delipidized algal residue

There are additional features and advantages of the various embodiments of the present disclosure. They will become evident from the following disclosure.

In this regard, it is to be understood that this is a summary of the various embodiments described herein. Any given embodiment of the present disclosure need not provide all features noted above, nor must it solve all problems or address all issues in the prior art noted above.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments are shown and described in connection with the following drawings in which:

FIG. 1 is a schematic diagram illustrating an embodiment according to the present disclosure of processing an algae-containing feedstock using hydrothermal carbonization.

FIG. 2 is a schematic diagram providing a flowchart for a process for assessing the durability of pelletized materials.

FIG. 3 is table summarizing water immersion pellet durability tests of pellets made from whole Spirulina.

FIG. 4 is table summarizing water immersion pellet durability tests of pellets made from lipid-extracted Spirulina.

FIG. 5 is table summarizing water immersion pellet durability tests of pellets made from whole Spirulina treated via HTC at 175° C.

FIG. 6 is table summarizing water immersion pellet durability tests of pellets made from lipid-extracted Spirulina treated via HTC at 175° C.

FIG. 7 is a graph of mass recoveries, elemental analyses, aqueous coproducts (“ACP”), and gasses produced from HTC treatment of lipid-extracted algae (“LEA”) and whole Spirulina compared with loblolly pine and sugarcane bagasse (the balance of the solids is equal to 100%).

FIG. 8 is photographs of raw Spirulina (A), hydrochar produced from Spirulina feedstock (B), and hydrochar produced from loblolly pine feedstock (C).

FIG. 9 is a table showing hydrochar recoveries and compositions for various biomass feedstocks, including feedstocks according to various implementations of an embodiment of the present disclosure.

FIG. 10 is a graph illustrating the energy density of algal feedstocks (stars) compared with various lignocellulosic feedstocks (squares) at various HTC reaction temperatures and a 30 minute hold time.

FIG. 11 is a graph showing inorganic elemental analysis by X-ray fluorescence of feedstocks and hydrochars produced therefrom, according to an embodiment of the present disclosure, expressed as a percentage of starting dry mass (not including C, H, N, and O).

FIG. 12 is a table listing value added chemicals that may be produced from biomass.

FIG. 13 is a table showing the compositions of aqueous co-products formed during HTC of various algal-containing feedstocks according to an embodiment of the present disclosure, and various lignocellulosic feedstocks.

FIG. 14A is a graph showing the results of GC/MS analysis of polar compounds in aqueous products resulting from HTC treatment of whole and LEA Spirulina at 175° C. according to an embodiment of the present disclosure.

FIG. 14B is a graph showing the results of GC/MS analysis of sugars and sugar alcohols in aqueous products resulting from HTC treatment of whole and LEA Spirulina at 175° C. according to an embodiment of the present disclosure; species that are identified as high value chemicals are outlined.

FIG. 15 is a graph of the results of HPLC-RI analysis of sugar in aqueous products from HTC treatment of various feedstocks (including according to an embodiment of the present disclosure), expressed as a percent of starting dry mass; sugars noted as high value chemicals are outlined.

FIG. 16 is a graph showing high value chemicals, as a percentage of starting dry feedstock, identified from HPLC and GC/MS analysis of sugars in the aqueous fraction from HTC of Spirulina at 175° C. for 30 minutes, according to an embodiment of the present disclosure.

FIG. 17 is a schematic diagram of a HTC process, and product collection, according to a particular example of an embodiment of the present disclosure.

DETAILED DESCRIPTION

Unless otherwise explained, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. The singular terms “a,” “an,” and “the” include plural referents unless the context clearly indicates otherwise. Similarly, the word “or” is intended to include “and” unless the context clearly indicates otherwise. The term “includes” means “comprises.” The terms “solvent,” “a solvent” and “the solvent” include one or more than one individual solvent compound unless indicated otherwise. Mixing solvents that include more than one individual solvent compound with other materials can include mixing the individual solvent compounds simultaneously or serially unless indicated otherwise. The separations and extractions described herein can be partial, substantial or complete separations unless indicated otherwise. All percentages recited herein are weight percentages unless indicated otherwise. All numerical ranges given herein include all values, including end values (unless specifically excluded) and intermediate ranges.

FIG. 1 illustrates a general method 100 for converting algae to biofuel. In step 105, algae is harvested or otherwise obtained. The algae may be from any suitable strain or combination of strains. Strain selection may take various factors into account, including ease of growth and processing, lipid content, and reaction products and byproducts. For example, hydrothermal carbonization (HTC) typically produces hydrochar and an aqueous phase that includes various reaction products/byproducts, such as high value organic compounds and sugars.

After harvesting, algae is typically dewatered in step 110. Dewatering may be carried out through mechanical, thermal, or chemical means. Initial dewatering techniques can include centrifuges, decanters, filters, hydrocyclones, mechanical presses, and flocculation, including polymer flocculation. If a greater degree of drying is desired, drying can include direct heat drying, fluidized bed dryers, microwave dryers, or steam drying. In some implementations, the algae is dried to a biomass (algae) concentration of 98% or less by weight, such as less than about 95%, about 90%, about 50%, about 30%, about 25%, about 20%, about 15%, about 10%, about 7%, about 5%, or about 2%. In other implementations, the algae is dried to a biomass concentration of between about 1% and about 30% by weight, such as between about 2% and about 25%, between about 1% and about 10%, between about 2% and about 7%, between about 2% and about 15%, or between about 7% and about 15%. In other implementations, the dewatering or drying process is omitted.

An advantage of at least certain implementations of the present disclosure is that that the HTC process may employ slurries of algae, which may not require extensive dewatering or drying. In at least one example, a slurry is feedstock having about 2% to about 20% by weight of biomass, such as algae or delipidized algal residue, in a liquid carrier, such as water. In a more specific example, the slurry has a biomass content (such as algae or delipidized algal residue) of between about 2% and about 7% or between about 5% and about 20% by weight. In another implementation, the present disclosure uses feedstocks that contain a higher percentage of biomass, such as feedstocks in the form of wet pastes.

After harvesting, and optionally drying, the algae, lipids are optionally extracted from the algae in step 115. Suitable methods of removing lipids from the algae include, without limitation, expellers, presses, solvent extraction, supercritical carbon dioxide extraction, enzyme extraction, and ultrasonication.

The extracted lipids can be optionally processed in step 120. For example, the lipids may be converted to a biofuel product, such as biodiesel, as is known for lipids obtained from other sources. One suitable method of converting lipids to a biofuel product is transesterification.

The feedstock for the method 100 is typically a mixture of solids and a liquid carrier, typically water, for reaction. In a specific example, the mixture is a slurry. The components of the feedstock can be modified prior to reaction in step 125. For example, water or another carrier can be added to produce a desired amount of solids in the slurry. Additional feedstocks 130, such as from other biomass sources, may be added to the slurry. Additional components may be added, such as to influence the reaction rate or reaction products. For example, acid, such as acetic acid, can be added, which may assist in suppressing gas formation and favoring solid products.

In some implementations, the biomass component of the feedstock consists essentially of algae with the lipids still present. In other implementations, the biomass component of the feedstock consists essentially of delipidized algae. The biomass component, in a further implementation, consists essentially of a mixture of algae and delipidized algal residue.

In some examples, a biomass component “consisting essentially of” a particular component, or mixture of components, means that the feedstock does not include a significant portion of biomass other than from the recited sources. In a particular example, biomass from other sources is less than about 10% by weight of the biomass component of the feedstock. In another example, biomass from other sources is less than about 5% by weight of the biomass component of the feedstock. In yet another example, biomass from other sources is than about 1% by weight of the biomass component of the feedstock.

In a further example, which may, but is not required to, be combined with the preceding examples, the feedstock can include additional components, such as salts, solvents, pH modifiers, and similar components that affect the process parameters of the HTC reaction or the products produced therefrom.

In further implementations, the biomass component of the feedstock is at least about 5% by weight of algal origin (algae or delipidized algal residue), such as being at least about 10%, about 25%, about 50%, about 75%, about 85%, about 90%, or about 95% of algal origin.

In some examples, the amount of feedstock (the algal feedstock combined with any other feedstock) in the aqueous slurry is between about 1% and about 50% by weight solids, such as between about 2% and about 20% solids, between about 2% and about 15% solids, between about 2% and about 10% solids, between about 2% and about 7% solids, between about 5% and about 40% by solids, between about 10% and about 30% solids, and about 15% and about 20% solids. In further examples, the solids are at least about 1% by weight of the slurry, such as at least about 2%, about 5%, about 10%, about 15%, about 20%, about 25%, about 30%, about 40%, or about 50% solids. In other examples, the solids are less than about 50% by weight of the slurry, such as being less than about 40%, about 30%, about 20%, about 10%, or about 5% of the slurry.

The algae, or lipid-extracted algal residues, are subjected to a hydrothermal carbonization reaction in step 135. Hydrothermal carbonization reactions typically take place at elevated temperatures and pressures. In one implementation, in various examples, suitable temperatures are between about 120° C. and about 295° C., between about 120° C. and about 250° C., between about 155° C. and about 295° C., between about 150° C. and about 250° C., between about 180° C. and about 300° C., between about 180° C. and about 250° C., between about 220° C. and about 250° C., between about 225° C. and about 250° C., between about 190° C. and about 240° C., or between about 200° C. and about 220° C. In other examples, the temperature is less than about 250° C., such as less than about 240° C., about 230° C., about 220° C., about 210° C., about 200° C., about 190° C., about 180° C., about 170° C., about 160° C., or about 150° C.

Suitable pressures are typically between about 2 bar and about 175 bar, such as between about 2 bar and about 100 bar, between about 5 bar and about 85 bar, between about 5 bar and about 40 bar, between about 10 bar and about 85 bar, between about 10 bar and about 40 bar, and between about 20 and about 30 bar. In further examples, the pressure is less than about 85 bar, such as less than about 40 bar, about 30 bar, about 20 bar, or about 10 bar. In some implementations, the pressure is sufficient to maintain water in the reactor in a condensed state, such as a pressure that is at least equal to the saturated vapor pressure of water at the reaction temperature.

In some cases, the temperature of the reaction mixture is controlled, while the pressure is the autogenous pressure produced by the reaction mixture at that temperature. In other cases, the temperature and pressure are independently controlled. For example, the reaction may be carried out in an overpressure environment.

In further implementations, the HTC process is carried out at a temperature of between about 150° C. and about 250° C., such as between about 175° C. and about 225° C. In these implementations, the pressure is at least about the saturated steam pressure at the temperature. In one example, the pressure is about the saturated steam pressure at the temperature.

In some cases, the HTC reaction is carried out in an ambient atmosphere. In other cases, the atmosphere is reduced in oxygen or another component. For example, the atmosphere, such as prior to reaction, may be fully or partially purged with an inert gas, or otherwise unreactive gas, such as nitrogen, helium, argon, or mixtures thereof.

The HTC process is typically carried out for a period of time for a sufficient level of conversion to take place. The duration of the HTC process may be influenced by a number of factors, including the nature of the algal feedstock (including whether the algae is present with lipids intact or as delipidized residue), the temperature selected for the reaction, and the pressure selected for the reaction. The temperature can be selected to provide a desired reaction rate. The relationship can be evaluated according to the “severity factor”, given by:

Severity Factor=Log10 (time*exp[(Temp.−100)/14.75])   (1)

The Severity factor is further described in Overend, et al., Fractionation of Lignocellulosics by Steam-Aqueous Pretreatments. Phil. Trans. R. Soc. Lond., A 321, 523-536. 1987, incorporated by reference herein to the extent not inconsistent with the present disclosure. Generally, higher temperatures require shorter processing times to achieve a similar level of conversion.

The HTC process parameters, including, temperature, pressure, and reaction time, may also be selected based on process efficiency for a desired output, as well as the nature of the desired output. For example, the process conditions may be adjusted to favor the production of hydrochar or certain components present in the liquid carrier after the reaction. The presence of co-feedstocks may also influence processing conditions and times.

In some examples, the reaction is carried out for between about 5 minutes and about sixteen hours, such as between about 5 minutes and about six hours, between about 5 minutes and about four hours, between about 5 minutes and about 3 hours, or between about 4 hours and about 16 hours. In further examples, the reaction is carried out for between about 5 minutes and about 180 minutes, between about 15 minutes and about 120 minutes, between about 30 minutes and about 90 minutes, between about 5 minutes and about 50 minutes, or between about 5 minutes and about 30 minutes. In other examples, the reaction is carried about for at least about 5 minutes, such as at least about 10 minutes, about 15 minutes, about 30 minutes, about 45 minutes, about 60 minutes, about 90 minutes, about 120 minutes, or about 180 minutes. In yet further examples, the reaction is carried out for less than about 240 minutes, such as less than about 180 minutes, about 120 minutes, about 90 minutes, about 60 minutes, about 45 minutes, about 30 minutes, about 15 minutes, or about 10 minutes. In other examples, the reaction is carried out for a different amount of time.

For example, when the reaction is carried out in an extrusion system, the reaction may be carried out in less than about thirty minutes, such as less than about 20 minutes, less than about 10 minutes, less than about 5 minutes, less than about 2 minutes, or less than about a minute. In more specific examples, the reaction may be carried out in less than about 60 seconds, such as less than about 45 seconds, less than about 30 seconds, less than about 20 seconds, or less than about 10 seconds.

After the reaction has reached a desired level of completion, the formed hydrochar is typically collected in step 140. Suitable methods of collection include decantation, filtration, and centrifugation.

In some implementations, the hydrochar is converted to a compressed solid, such as being pelletized or formed into a briquette, in step 145, which may aid in using or transporting the fuel. Surprisingly, it has been found that hydrochar formed from algae, whether the feedstock includes or does not include lipids, is capable of forming compressed solids, such as pellets or briquettes, without added binder. Typically, when hydrochar is formed from lignocellulosic biomass, lignin present in the hydrochar can serve as a binder for the compressed solid. However, algae lacks lignin, and thus would have been expected to require an added binder. However, a binder can be included in some implementations, if desired.

Prior to conversion to a compressed solid, the hydrochar may be processed to aid in the compression process. For example, the hydrochar may be ground or chipped to a more uniform size or size range. Typically, the particles are reduced to a size that is less than the thickness of the compressed solid that will be produced. In a specific example, the material used to create a compressed solid has an average cross-sectional width of less than about 3 mm. In another example, the material to be pelletized is in the form of a powder, such as a powder having an average cross-sectional width of less than about 1 mm, such as less than about 0.75 mm, less than about 0.5 mm, or less than about 0.25 mm.

In some implementations, the compressed solid is a pellet. In various examples, the pellets have an average cross sectional width of between about 3 mm and about 24 mm, such as between about 6 mm and about 8 mm, between about 10 mm and about 12 mm, or between about 6 mm and about 10 mm. In a specific example, the pellets have an average cross sectional width of about 13 mm.

In further implementations, the compressed solid is a briquette. In a particular example, the briquettes have an average cross sectional width of at least about 25 mm.

In some aspects of the disclosed method, the hydrochar is compressed into a form other than pellets or briquettes, or is not compressed.

The hydrochar may also be cleaned, such as by rinsing with solvents or by screening, to remove contaminants or aid in processing. The hydrochar may also be treated, such as with water or steam at varying temperatures. The hydrochar may also be dried prior to pelletization (or other uses). In some examples, the hydrochar is dried to less than about 10% moisture by weight. In other examples, the hydrochar has a moisture content of less than about 15% by weight, such as less than about 12% by weight, less than about 10% by weight, less than about 7% by weight, or less than about 5% by weight. In further examples, the hydrochar is dried to a moisture content of between about 1% and about 10% by weight, such as between about 2% and about 8% by weight, between about 3% and about 7% by weight, between about 7% and about 9% by weight or between about 8% and about 12% by weight. In some aspects of the disclosed method, the hydrochar is not dried.

In some implementations, the temperature of the hydrochar feed, the pelletization apparatus, or the environment of the pelletization apparatus is selected to influence the pelletization process or pellet properties. For example, the die through which the hydrochar is extruded may be heated or cooled. In a specific example, the die is operatively associated with an induction heater, such as a band heater. In another specific example, a coolant, such as water, is applied to the die.

In certain examples, the extrusion/pelletization mechanism is operated at a temperature of between about 0° C. and about 200° C., such as between about 0° C. and about 180° C., between about 0° C. and about 50° C., between about 25° C. and about 180° C., between about 25° C. and about 150° C., between about 75° C. and about 150° C., between about 75° C. and about 125° C., between about 75° C. and about 100° C., or between about 100° C. and about 150° C. In further examples, the temperature is less than about 200° C., such as less than about 180° C., about 150° C., about 125° C., about 100° C., about 75° C., or about 50° C. In a specific example, the pelletization temperature is selected to be less than that typically required for pelletization of hydrochar produced from lignocellulosic biomass sources.

The force applied to the hydrochar during pelletization may also affect the properties of the pellets, in addition to altering the pelletization conditions. The pressure is typically selected to be in a range that produces pellets of a desired hardness; being sufficient to bind particles together, but not so great as to shear the bound particles apart. In certain implementations, a pellet press, such as the Carver bench top laboratory hydraulic press, Model M (available from Carver, Inc., of Wabash, IN) is set to a pressure of between about 0.5 MPa and about 250 MPa, such as between about 5 MPa and about 250 MPa, between about 5 MPa and about 50 MPa, between about 10 MPa and about 50 MPa, between about 15 MPa and about 40 MPa, such as between about 50 MPa and about 200 MPa, between about 75 MPa and about 175 MPa, or between about 100 MPa and about 150 MPa. In other implementations, the pressure is less than about 250 MPa, such as less than about 225 MPa, about 200 MPa, about 175 MPa, or about 150 MPa. In further aspects of the disclosed method, other pressures are used.

In another aspect of the invention, the pressure applied to the surface of the pellet-in-formation is between about 30 MPa and about 750 MPa, such as between about 60 MPa and about 500 MPa, between about 100 and about 400 MPa, or between about 150 MPa and about 250 MPa. In other examples, other pressures are used.

The duration the pellet-in-formation is subjected to compression may also influence pellet properties and pelletization times, with increased times generally resulting in harder pellets and longer production times/lower throughput. The hold time is typically selected to provide the appropriate balance of hardness and throughput, as desired by an operator. In one implementation, the hold time is between about 1 second and about 600 seconds, such as between about 1 second and about 60 seconds, between about 5 seconds and about 60 seconds, between about 15 seconds and about 60 seconds, or between about 30 seconds and about 45 seconds. In other examples, the hold time is less than about 600 seconds, such as less than about 180 seconds, less than about 90 seconds, less than about 60 seconds, less than about 45 seconds, or less than about 30 seconds. In other aspects of the disclosed method, other hold time are used.

In some implementations, the algae hydrochar, whether or not from delipidized, or algae/co-feed hydrochar, is processed into pellets without the addition of external binders. In other examples, a binder is added to the hydrochar prior to pelletization. Suitable binders include lignin, paraffin oils, starches, fats, proteins, sugars, or mixtures thereof. In specific examples, the amount of binder is less than about 10% by weight of the material to be pelletized, such as less than about 5%, about 4%, about 2%, or about 1%. In a particular example, the amount of binder is between about 2% and about 4% by weight of the material to be pelletized. In further aspects of the disclosed method, other binder percentages are used.

In another example, the hydrochar from the HTC reaction is combined with hydrochar produced from HTC of lignocellulosic feedstock. The lignocellulosic hydrochar can assist in pelletizing the algal-sourced hydrochar. In yet another example, the hydrochar produced from an embodiment of the present disclosure is used as a binder for raw lignocellulosic biomass, torrefied or pyrolyzed biomass, or hydrochar from biomass sources other than from an embodiment of the present disclosure.

As shown in step 150, the method 100, in some implementations, includes adding additional feedstocks to the material to be compressed. The feedstocks can include, for example, raw biomass, such as unprocessed lignocellulosic biomass, torrefied or pyrolyzed biomass (such as lignocellulosic biomass), or hydrochar produced through hydrothermal carbonization of a different source than the particular feedstock 125 used in the example. In another aspect, the additional feedstock 150 includes coal fines or pulverized coal. The additional feedstock 150, in one example, is another feedstock produced according to an embodiment of the present disclosure. In another example, the feedstock 150 is a feedstock produced other than through an embodiment according to the present disclosure. Mixtures of additional feedstocks 150, including both those produced according to a method of the present disclosure and those not so produced, are used in yet another example.

The pellet mill, extruder, or other pelletization apparatus may be set to produce pellets of a desired size, hardness, and quality, including by selecting a suitable pellet die thickness, processing speed, temperature, and pressure. The exact nature of the pellets and processing conditions may be affected by the desired use. For example, softer, less robust, but more quickly processed pellets may suffice when the pellets are to be used quickly. Harder pellets may be indicated when the pellets are to be transported or stored for long periods of time, or when the environment to which the pellets will be exposed warrants harder pellets. For example, environment having higher humidity may benefit from harder pellets.

Suitable pellet mills include those available from Farm Feed Systems Ltd. of Gloucestershire UK, Pellet Pros of Dubuque, Iowa, Andritz AG of Graz, Austria, California Pellet Mill of Crawfordsville, Ind., and Amandus Kahl GmbH & Co. KG, of Reinbek, Germany. Pellet mills types include, without limitation, flat die pellet mills and ring die pellet mills.

After the pelletization process, the pellets are typically cooled. The pellets may also be screened, such as to remove residual fines. A drying process, with or without heat, may also be carried out. In a specific example, air is blown through the pellets to achieve a final moisture content of less than about 10%, such as less than about 8%, less than about 6%, less than about 4%, or less than about 2%. In another example, the moisture content is between about 2% and about 15%, such as between about 4% and about 10% or between about 4% and about 8%.

In other embodiments, in step 155, the hydrochar from reaction 125, such as that collected in step 140, is directly gasified, liquefied, or otherwise processed, such as without prior pelletization.

In some implementations of the present disclosure, rather than being compressed into a solid, the hydrochar is extruded. In one example, the hydrochar is extruded after being formed. In another example, the hydrochar is formed and extruded in a common process. For example, the extrusion process may generate sufficient temperatures and pressures to carry out hydrothermal carbonization of the feedstock prior to be being extruded. One suitable extrusion apparatus and method is described in U.S. Patent Publication US 2014/0262727, incorporated by reference herein to the extent not inconsistent with the present disclosure.

EXAMPLE 1

Pelletization of HTC Hydrochar from Algal Feedstocks

In some cases, production of satisfactory pellets is assisted when the solid material is heated while being compressed. Heating can assist the binder (either natural or added) in becoming fluidized, causing more effective adhesion of the particles. In this Example, a heated die system was used. The system included a 13-mm diameter, hardened steel heated die (Across International, Berkeley Heights, N.J.), along with an Omega bench top temperature controller (Model CSC32J) and Omega thermocouple (type J iron-constantan).

The heated die system used to create pellets consisted of a 13 mm diameter die with an electric heating element, support plate, core dies, thermal insulator plate, push rod, and pellet ejector. Schematics of this die system are available at the website of Across International, including, for example, http://www.acrossinternational.com/13mm-1-2-Diameter-ID-250C-Heated-Die-w-Digital-Controller-SDS13H.htm, and are incorporated by reference herein. To produce a pellet, the die is first heated to the desired temperature (such as, in one example, 140° C.). The die is then placed on its support plate and a steel core die is inserted from the top. Approximately 1 g of hydrochar (or other material to be pelletized) is added on top of the core die. The push rod is inserted and the die assembly is placed on the press.

A Carver bench top laboratory press Model M (Menomonee Falls, Wis.) was used to produce pellets from raw and hydrotreated biomass materials. The pressure gauge (Wika Instruments Model 232.34, 0-5000 psi) enabled accurate determinations of hydraulic pressure, thus improving the uniformity of produced pellets. The handle of the press was manually depressed while watching the pressure gauge. A hydraulic pressure of 20 MPa (equivalent to about 295 MPa at the surface of the pellet-in-formation) was attained and held for 60-seconds. The pressure was then released and the die assembly removed from the press. The die body was removed from the support plate base and placed on the pellet ejector base. This assembly was then placed back on the press, and gentle pressure applied to the push rod until the pellet ejected out the bottom.

Pellet lengths were measured with a set of Vernier calipers. Assuming a diameter of 13 mm, these measurements were used to calculate the volume of an individual pellet. Knowing the pellet volume enables calculation of both mass density (kg/m³) and energy density (GJ/m³) of individual pellets.

Pellet Durability Testing

Several standard (and non-standard) tests are commonly employed to evaluate the quality of biomass pellets. These tests address properties such as mass density, energy density, compressive strength, durability, modulus of elasticity, equilibrium moisture content, and others.

A series of tests were carried out to systematically explore the water tolerance behavior of pellets. The aim was to develop standard tests that could be utilized to quantify water tolerance, thereby enabling meaningful comparisons among different types of pellets—including pellets produced from blends of hydrochar with other materials such as raw biomass, torrefied biomass, and coal.

The approach followed is summarized in the schematic of FIG. 2. A traditional tumbler test was used to define pellet durability, both before and after immersion in water for varying lengths of time. The apparatus used was a Thumbler' s Model A-R1 rotary tumbler, with a 4½-in. rubber barrel. In the standard test, 40 pellets were placed in the barrel and rotated for 3000 revolutions at a speed of approximately 38 rpm (typically over a period of approximte 90-min. of tumbling).

Durability was defined as the ratio of pellet weight after tumbling to the initial pellet weight. Weight determinations were made using an Acculab Model ALC80.4 analytical balance, with a sensitivity of +/−0.0001 g.

Water immersion test results show a high degree of repeatability, and provide a means for readily distinguishing among pellets that exhibit different water-immersion behaviors. The water immersion and tumbling process is an extremely severe test of pellet durability, and only very robust pellets can maintain their integrity when exposed to these conditions. In cases where the pellets are very robust, 3 test pellets and 37 round wood filler pellets may be used in a single tumbler test, and to determine the individual weight loss from each pellet. However, low-stability pellets often lose a significant fraction of their weight, due to attrition of large fragments, making it difficult to identify the same pellet before and after tumbling if multiple pellets are used in a single tumbler test. Thus, for these weaker pellets, tumbler testing of a single pellet along with 39 filler pellets may be used.

Tests were carried out to determine whether reliable durability results could be obtained using a smaller number of pellets. In these tests, spherical objects were substituted as “filler” for most of the 40-pellets, and only a small number of actual test pellets were used. Three different filler materials were investigated −½ in. diameter solid balls of maple wood, low-carbon steel, and plastic (HDPE) (all three of these filler types were obtained from McMaster-Carr).

Hydrochar pellets were severely damaged when steel balls were used in the tumbler test, resulting in low values of pellet durability. The wood and plastic filler materials behaved similarly, and resulted in much less pellet damage. Because the wood balls are more similar to hydrochar pellets in material composition and density, wood fillers were used in subsequent tumbler tests.

Pellets were made from whole Spirulina feedstock, lipid-extracted Spirulina feedstock, whole Spirulina treated via HTC at 175° C., and lipid-extracted Spirulina treated via HTC at 175° C. Pellets were immersed for 0 minutes (control sample) and 60 minutes. These tests were conducted in triplicate. Before immersion, each pellet was weighed and its dimensions were measured. From these measurements, pellet densities were calculated, expressed as kg/m3. After water immersion, the pellets were allowed to air dry for twenty-four hours, and were then re-measured for weight and length. In addition, each pellet underwent a tumbler durability test, along with 39 round wood filler pellets as described above. Results of these tests are summarized in tabular form in FIGS. 3-6.

As shown in FIG. 3, pellets of whole Spirulina, with lipids intact, exhibit nominal weight changes before water immersion and after 60 minutes of water immersion. Pellet length exhibited a minor increase. However, while unimmersed pellets generally exhibit high durability, typically exceeding 90%, water-immersed pellets lost structural integrity during the tumbling process, resulting in the tumbled pellets having signficantly lower weights than before tumbling. The pellet durablity for the immersed pellets was typically less than 60%, such as being less than 55%.

Referring to FIG. 4, pellets of deplipidized Spirulina residue exhibited minor changes in weight and length before and after water immersion for 60 minutes. Like the pellets formed from whole Spirulina, pellets from delipidized Spirulina residue were very stable in the tumble test, pre-immersion. After 60 minutes of water immersion, the tumbled pellets again exbhibited significant degradation and weight loss. The pellet stability for pellets from delipidized Sprirulina residue was typically less than about 60%, such as less than about 55%.

FIGS. 5 and 6 present the results of immersion test for whole Spirulina and delipidized Spirulina residue, respectivly, treated via HTC at 175° C. Like the unprocessed source materials, the hydrochar material exhibited only minor changes in weight and length before and after water immersion. However, the hydrochar pellets were substantially more stable, even after water immersion, than pellets from the unproccessed source material. Pellets formed from HTC of whole Spirulina exhibited greater than 75% stability even after water immersion, with stabilities typically being around 80%. Even pellets formed from HTC of delipidized Spirulina residue exhibited enhanced stabilities of greater than 65%, even after water immersion, with typically stabilities being between about 70-80%.

EXAMPLE 2

Hydrothermal Carbonization of Algal Feedstocks

HTC processes were conducted at 175° C. using both whole and LEA Spirulina, and at 215° C. for whole Spirulina. Results of these processes were compared with results from treatment of lignocellulosic feedstocks, using examples of loblolly pine and sugarcane bagasse.

A mass balance of each HTC experiment was computed by determining the mass of each recovered product and comparing the sum of all products recovered to the total dry starting mass. The recovered products include the solid hydrochar, gases (mainly CO₂ with small amounts of CO), aqueous co-products (ACP), and produced water. Very little water is typically produced under the low process temperature conditions used in this Example 2.

The mass recoveries from Spirulina experiments are shown in FIG. 7, along with recoveries from loblolly pine and sugarcane bagasse feedstocks for comparison. The composition of the feedstock was normalized to 100%, and the three product bars (hydrochar, ACP and gas shown as the offset bars) show the percentage mass recovery of each so that the sum of the three show the total mass recovery of the starting dry feedstock. The relative composition in terms of C, H, N, S, O, and ash are illustrated for both the starting dry feedstock and the recovered hydrochar by the shaded, stacked bars. The balance of mass is shown when the compositional elements do not add up to 100% (Note that oxygen is measured directly). The total mass that was recovered in the aqueous co-product (ACP) and gaseous phases are represented by the offset bars.

FIG. 7 illustrates that much lower mass fractions were recovered as hydrochar from the algae experiments as compared to the lignocellulosic feedstocks, and that much greater mass was recovered in the ACP. At 175° C., less than 50% of the starting mass was recovered as hydrochar from both LEA and whole Spirulina, while hydrochar recoveries from lignocellulosic feedstocks were greater than 70%. Hydrochar recovery was further reduced with increasing temperatures, with a larger effect seen for algae compared to the lignocellulosic feedstocks.

FIG. 7 also shows that much less of the carbon in the starting feedstock was recovered in the algae hydrochar in comparison with the lignocellulosic hydrochars. About 50% of the carbon is retained in the solid hydrochar from algae at 175° C., while 80%-90% is retained after HTC treatment of lignocellulosic feedstocks. Note also that the oxygen contents of the algae hydrochar were reduced significantly, similar to the lignocellulosic hydrochar. In addition, much of the ash constituents in the algal feedstocks were solubilized in the water, and are significantly reduced in the resulting hydrochar. Taken together, these compositional changes result in an energy densified solid, as discussed in the next section.

Much of the starting algal mass was recovered as non-volatile residue (NVR) after HTC treatment, which was measured through oven drying of the ACP. The ash fraction of the solid feedstock that was washed into the aqueous phase contributes to this NVR, along with other nitrogen-containing Maillard-type heterocyclic compounds and piperazinediones. In a similar trend to the lignocellulosic feedstocks, the mass recovered as NVR was reduced as treatment temperature increased. This is primarily due to increases in the production of volatile compounds such as formic acid, acetic acid and furfural. Note that the only portion of ACP included in FIG. 7 is the NVR; other volatiles that may be lost through oven drying are not included. Similar to treatment of lignocellulosic feedstocks, only a small amount of gas (primarily CO₂) is produced at low HTC treatment temperatures.

At an HTC treatment temperature of 175° C., nearly all of the starting algal mass is accounted for by the three recovered products. However, as the treatment temperature is increased to 215° C., only 85% of the starting mass is accounted for. This could be due to higher amounts of water being produced (note the reduction in hydrogen), or from greater production of volatiles that were not measured, such as ammonia.

Hydrochar Products

HTC of algal feedstocks produces a hydrophobic char that is easily dried and pelletized. Photographs of the Spirulina feedstock and resulting hydrochar products are shown in FIG. 8, along with a photo of loblolly pine hydrochar. Results from characterization of the feedstocks and hydrochars are given in FIG. 9. Energy densification is defined as the energy content of the hydrochar divided by that of the starting feedstock (both on a dry basis). Energy yield is then the mass yield multiplied by the energy densification.

The energy content of the raw algae was similar or even higher than that of woody feedstocks treated previously (e.g., loblolly pine). In addition, the energy densification seen, even at these low temperatures, is much higher than for comparable treatment temperatures of lignocellulosic feedstocks. In earlier studies, very little energy densification of lignocellulosic hydrochar was seen at treatment temperatures less than 200° C. For algal feedstocks, however, energy densification of around 1.1 occurred at 175° C., while densification of 1.3 was observed at 215° C. The energy densification of Spirulina at 215° C. is equivalent to that observed from lignocellulosic feedstocks at temperatures of 255° C. or higher. Thus it appears that these algal materials can be converted to hydrochars under considerably milder HTC process conditions than required for treatment of lignocellulosic feedstocks. This is attributed in part to the lack of cellulose and lignin structures in algae (which are difficult to break down), and to the presence of high energy lipids. However, because of the low hydrochar mass recovery from algae, the overall energy yield in algal hydrochar is much lower than in lignocellulosic hydrochar.

The elemental compositions of the biomass feedstocks and hydrochar products are given in FIG. 9. The algal feedstocks have much lower oxygen contents than the lignocellulosic feedstocks. Consequently, the atomic O/C ratio for algae is approximately 0.4, as compared to 0.7 for lignocellulosic biomass. HTC treatment of whole Spirulina at 215° C. produced a hydrochar having an O/C ratio of 0.22, which approaches that typically associated with lignite or bituminous coal.

The energy contents of the biomass feedstocks and resulting hydrochars are shown in FIG. 10 for treatment of both whole and LEA Spirulina, along with results obtained from HTC treatment of lignocellulosic biomass. The algal feedstocks treated here have slightly higher starting energy contents than the lignocellulosic feedstocks. However, substantial energy densification of the algal hydrochars was observed at much milder process conditions than typically required when treating lignocellulosic feedstocks.

Elemental analysis was performed using X-ray fluorescence (XRF) (PANalytical, Westborough, Mass., USA) on the feedstock and hydrochar from each HTC experiment to evaluate the fate of the inorganic fraction in the algal feedstock. The results are expressed as a percentage of starting dry mass and shown in FIG. 11. Much of the ash constituents that are present in the starting feedstock are not seen in the solid product, indicating that the HTC process is effective in extracting some of them into the aqueous phase. At 175° C., 80% of the inorganic fraction is removed from both whole and LEA Spirulina, while at 215° C., 92% is removed.

This includes elements such as chlorine (10%-20%), magnesium (5%-50%) and calcium (25%-40% reduction), which have adverse effects during combustion. HTC may also result in a reduction in inorganics from lignocellulosic feedstocks, ranging from 50% to 75% at temperatures of 200° C. Lower concentrations of silicon in Spirulina (about 0.3%) in comparison to lignocellulosic feedstocks (1.1%-3.6%), which is largely not removed by HTC, contribute to a larger reduction in the inorganic fraction seen here. This reduction in inorganic fraction also contributes to the energy densification of the hydrochar. FIG. 11 also suggests that some ash constituents were removed during the lipid extraction process. In particular, comparing the two feedstock bars indicates that significant fractions of sodium and magnesium were removed by extraction. However, it should be noted that the XRF method of evaluation for inorganics applied here is semi-quantitative for sodium and magnesium.

Aqueous Co-Products

To identify potential high-value chemicals in the ACP as shown in FIG. 12, a series of laboratory analyses were completed. A summary of these results is shown in FIG. 13 in comparison to similar results from HTC treatment of loblolly and sugarcane bagasse. Although much of the mass is recovered in the ACP as a non-volatile residue (NVR), only a small fraction of the mass is identified through multiple analyses applied. An analysis of the total organic carbon (TOC) of the ACP shown in FIG. 13, taken with the carbon content of the solids FIG. 9 and the total gases produced gives a carbon balance within 85%-90%. This suggests that the elemental analysis of the solids is useful to evaluate the nutrient content in the ACP. The reduction in nitrogen content of the solid hydrochar therefore indicates that much of the mass in the NVR is a result of other nitrogen-containing Maillard-type heterocyclic compounds and piperazinediones.

The pH of the aqueous co-products (ACP) was measured after each experiment and was found to be approximately 5.8, as shown in FIG. 13. This is considerably higher than the pH values of 3.0-3.5 that were seen from lignocellulosic feedstocks. Other volatiles, such as acetic and formic acid, were not measured in this study but are shown in FIG. 13 for comparison from lignocellulosic feedstocks. Higher pH may be related to the elevated N content of the algae feedstocks.

A gas chromatography/mass spectrometer (GC/MS) (Varian, Inc., Walnut Creek, Calif., USA) analysis was performed on the aqueous product streams from whole and LEA Spirulina treated at 175° C. to identify polar compounds and sugars or sugar alcohols. The polars results are shown in FIG. 14A; sugars/sugar alcohols are shown in FIG. 14B. In both cases, the results are expressed as a percentage of starting dry algal mass.

Using the analysis of polar compounds, malonic, succinic, and glutaric acids were detected in high concentrations relative to all species identified. However, less than 1% of the starting dry algal mass was converted into these identified species. From the sugars analysis, relatively large amounts of lactic acid were observed, with lesser amounts of trehalose and very small amounts of other sugar-related species. Although the high value sugars make up approximately 50% of the total sugars identified through this method, they are still a very small fraction of the starting dry feedstock. It is possible, however, that higher treatment temperatures would produce a greater amount of desirable chemicals. For example, maximum recovery of sugars from treatment of lignocellulosic feedstocks occurred around 230° C., while increasing amounts of acids (such as acetic and formic acid) were produced with increasing temperatures up to 295° C.

Interestingly, higher amounts of polar compounds were observed from HTC treatment of the whole algae, while approximately equivalent amounts of sugars were seen from HTC of whole and LEA Spirulina. This may be because the sugars are produced from degradation of carbohydrates (which are not removed by the extraction process used to obtain the LEA), while at least some of the polar compounds result from degradation of lipids (which are removed by extraction).

An HPLC-RI analysis (Waters Corporation, Milford, Mass., USA) was also applied to identify and quantify sugars in the aqueous products from HTC treatment of algae. The results are shown in FIG. 15, where they are compared with results from HTC treatment of woody and herbaceous feedstocks. Sugars that are identified as high-value chemicals are outlined in this figure (note that some of these sugars co-elute using this HPLC method). For experiments using these lignocellulosic feedstocks, treatment temperatures were varied from 175° C. to 295 ° C., although only temperatures of 235° C. and below are shown here, as they correspond more closely to the algal treatment temperatures. For the lignocellulosic feedstocks, produced sugars increased with treatment temperatures up to 235° C., and declined at higher temperatures. Sugars produced at low temperatures (175° C.) were primarily sucrose/trehalose, galactose/xylose/mannose, and fructose/inositol/arabinose. As temperatures increased, more glucose/pinitol, 5-HMF, and furfural were produced. 5-HMF and furfural are secondary products of cellulose degradation at these high temperatures. High value chemicals were produced in yields of 3%-4%, relative to the starting lignocellulosic feedstock mass. However, since several of the sugars co-elute, particularly those that dominate at low temperature conditions (e.g., fructose co-elutes with inositol and arabinose, and glycerol with mannitol), these yields of high-value chemicals may be slightly over-estimated.

Also shown in FIG. 15 are results from HTC studies with another algae, Scenedesmus Dimorphus. Although, not explored in detail, HPLC analyses of sugars from HTC treatment of Scenedesmus at three temperatures were performed. These results are shown in FIG. 15 for comparison with the Spirulina results. Clearly, these two algae materials produced different concentrations and types of sugars, although it should be noted that most of the HTC treatments of Scenedesmus were conducted at higher temperatures than those used for Spirulina. HTC of Scenedesmus produced higher yields of high-value sugars, primarily levoglucosan, arabitol, glycerol (which co-elutes with mannitol), and fructose (which co-elutes with inositol and arabinose). Similar high-value chemicals were produced by treatment of Spirulina at 215° C., although in lower yields. The low process temperature of 175° C. used in this Example 2 resulted in very low recovery of sugars from both whole and LEA Spirulina algae. The total mass of sugars recovered from both algae was much lower than that produced from the woody and herbaceous feedstocks. The products of cellulose degradation (furfural and 5-HMF) which dominate the total sugars from lignocellulosic feedstocks are largely absent from the algae products.

A compilation of results of identified high value chemicals from each of the methods described above is shown in FIG. 16. This illustrates that only a small fraction of the starting dry algae mass is converted to high value chemicals at these low process temperatures. Due to higher concentrations of malonic acid, HTC treatment of whole Spirulina resulted in nearly twice the amount of total valuable products as did treatment of LEA Spirulina. The amounts of other high-value products identified are similar from both algal feedstocks. The primary valuable products are glycerol/mannitol, arabitol, levoglucosan, lactic acid, malonic acid, and succinic acid.

Feedstock Preparation

Spirulina maxima was purchased in powdered form as a health supplement to evaluate for this Example 2. Spirulina typically contains 6%-13% lipids, 64%-74% proteins, and 15%-20% carbohydrates.

To obtain the LEA fraction, samples of whole, oven-dried algae were extracted using dichloromethane and hexane in an accelerated solvent extraction (ASE) instrument. 10.0% of the dry mass of the Spirulina was extracted through this process. The residues after lipid extraction are referred to as LEA.

The Scenedesmus Dimorphus that was evaluated previously was grown in outdoor ponds in Reno, (NV, USA). After harvesting, it was dewatered and frozen. The frozen wet algae were thawed at room temperature before use in the HTC process. Due to their growing conditions, the algae accumulated high concentrations of ash resulting from fertilizer use and dust contamination.

Hydrothermal Carbonization Reactor

Reactions were conducted in a 2-L Parr stirred pressure reactor (Model 4522, Parr Instruments, Moline, Ill., USA), as shown in FIG. 17. The reactor was charged with 50-60 grams of air-dried algal feedstock material, and 500-600 grams of distilled water in a 10:1 water to biomass ratio to ensure that all algae was thoroughly mixed with water to create a thin paste.

The vessel was first sealed, de-oxygenated (by flushing with helium), and then heated to the desired temperature while stirring. The reactor temperature was controlled with a National Instruments (Austin, Tex., USA) LabView data acquisition system. At the end of the reaction period, the reactor vessel was cooled by immersion in an ice bath, and the three product streams (gases, ACP, and solids) were isolated.

A vacuum filtration process is typically used to separate the solids from the ACP as illustrated in FIG. 17. However, because of the algae product's very small particle size, the filter paper quickly plugged up and slowed the vacuum filtration process. Therefore, a centrifuge process was used in which the solids and ACP were first separated at 6000 rpm for 30 min. Subsequent vacuum filtration was performed on the aqueous product to separate the fine particles.

Reaction Conditions

Effective carbonization of algae typically occurs at fairly mild temperatures. Maximum recovery of sugars often occurs at temperatures around 215-235° C. for a 30 minute reaction time. In an effort to maximize both high value chemicals and solid product recovery, a treatment temperature of 215° C. was initially selected with a 30 minute hold time. An initial run of whole Spirulina at 215° C. resulted in very low hydrochar recovery. Therefore, additional studies on whole and LEA Spirulina were completed with a target temperature of 175° C. to increase the recovery of the solid product.

Product Characterization

A variety of laboratory analyses were conducted on the HTC products to compute mass balance, carbon balance, and energy densification, as well as identify high value chemicals and other product species.

Hydrochar and Feedstock

Similar analyses were performed on the solid hydrochar and the feedstocks. The energy content of oven-dried samples was measured using a Parr 6200 calorimeter. Ultimate analysis (C, H, N, S, O) was performed using a Flash EA 1112 automatic elemental analyzer (ThermoElectron, Delft, The Netherlands). In order to directly measure the O content, two methods are used with two different injections, one to measure C, H, N and S, and the other to measure O.

To determine the amount of ash, proximate analysis was performed on the solid samples using a thermal gravimetric analyzer (Mettler Toledo TGA/DSC 1, Columbus, Ohio, USA). First, the samples were homogenized in an analytical mill (IKA ALL Basic, Wilmington, N.C., USA) for two minutes per sample. The homogenized samples were stored in capped glass vials at room temperature until analysis. The proximate analysis was then carried out according to ASTM standard D7582-12 with two differences; the volatile matter analysis was done at 700° C. instead of 950° C., and the sample size was limited to milligram amounts because the TGA instrument was equipped with small (70 μL) alumina crucibles. Two crucible blanks were analyzed for equilibration and subtraction of buoyance effects. Succeeding crucibles containing homogenized biomass samples were half filled to reduce surface area effects on pyrolysis. Each sample was analyzed in triplicate with every nine runs having an intermittent performance working standard (Vanguard Solutions VS6-006, Ashland, Ky., USA).

To perform the elemental analyses, solid particles were first re-suspended onto filters. In the re-suspension process, materials are first homogenized and then sieved to <38 μm diameter (400 mesh screen), then re-suspended using a high velocity air stream, blown into a large chamber for mixing and dispersion, and collected onto filters using a modified Parallel Impactor Sampling Device (PISD, OMNI Environmental, Portland, Oreg., USA). The filter samples are then analyzed using a PANalytical Epsilon 5 energy dispersive X-ray fluorescence (ED-XRF) instrument (PANalytical, Westborough, Mass., USA). The analyzer emits X-rays, which are focused on secondary targets and in turn emit polarized X-rays which excite a sample. Subsequent emissions of X-ray photons are integrated over time to give quantitative measurements of elements ranging from aluminum through uranium, and semi-quantitative measurements of sodium and magnesium.

Aqueous Co-Products

The pH and non-volatile residue (NVR) content of the ACP were measured immediately following completion of the reaction and separation of the products. The pH of the ACP was measured using a portable Hanna Instruments HI 8424 digital pH and temperature meter (Hanna Instruments, Smithfield, R.I., USA). The NVR content was measured by weighing triplicate samples of the ACP into drying tins which were then placed in a convective oven at 105° C. overnight (approximately 18-20 h) to obtain an oven-dried weight. The remaining residue represents the NVR content of the ACP.

The total organic carbon (TOC) and other sugars and polars were measured on a batch of samples once all experiments were completed. The ACP solutions were stored in a laboratory refrigerator until all samples were collected. The TOC was measured using a Shimadzu TOC-VCSH instrument (Columbia, Md., USA) which catalytically oxidizes all organic compounds into CO₂, which is then measured by nondispersive infrared detection (NDIR). Sugars were measured using a previously developed high-performance liquid chromatography (HPLC) method. Using a Waters Alliance 2695 HPLC (Waters Corporation, Milford, Mass., USA) equipped with a Waters 2414 Refractive Index Detector and Waters Sugar-Pak™ HPLC column, several sugars and sugar alcohols can be quantified, several of which are included in the United States Department of Energy's high value chemical list provided in FIG. 12. The high-value sugars identified through this method include furfural, levoglucosan and arabitol. Fructose and glycerol are also identified, although they co-elute with other sugars and are reported together.

Additional high-value chemicals were identified using GC/MS with two different analytical protocols: one called polars analysis and the other called sugars/sugar alcohols analysis. In both cases, the compounds of interest are extracted from the whole algae, LEA, and NVR fraction of the aqueous products from HTC treatment using the ASE instrument with dichloromethane, followed by acetone. After drying, the extracted materials are derivatized using BSTFA [N,O-bis-(trimethylsilyl) trifluoroacetamide] with 1% TMCS (trimethylchlorosilane). The derivatized samples are analyzed by an electron impact GC/MS technique using a Varian 3400 GC with a model CP-8400 autosampler and interfaced to a Saturn 2000 ion trap spectrometer (Varian, Inc., Walnut Creek, Calif., USA). A 30-m, DB-5 capillary column (0.25 mm ID; 0.25 μm thickness) was used for both analyses. For the sugars protocol, a set of calibration standards was used that consisted of numerous sugars, anhydrosugars, and sugar alcohols. For the polars protocol, a set of calibration standards was used that consisted of organic acids, lignin monomers, and other anhydrosugars.

Gases

The composition of produced gases was analyzed with an SRI 8610C gas chromatograph (GC, SRI Instruments, Torrance, Calif., USA), equipped with a thermal conductivity detector using a method for measurement of H₂, CO, CO₂, and C₁-C₃. The gases are comprised mainly of CO₂, and are not discussed in detail.

CONCLUSIONS

Hydrothermal carbonization (HTC) was applied to algae and lipid-extracted algae (LEA) residue to produce an energy-dense solid hydrochar that is similar in energy content to low-grade coals. Algal feedstocks behave differently in HTC treatment as compared to lignocellulosic feedstocks, and can benefit from milder conditions (treatment temperatures less than 200° C.) for acceptable levels of carbonization. These lower process temperature requirements result from the lack of lignin and cellulose structures in algae, which typically require higher process temperatures to break down in lignocellulosic feedstocks. However, a lower amount of the starting algal feedstock is recovered as a solid hydrochar, while more of the mass is recovered in the aqueous phase products. In part, the reduction of solid mass recovery and increase in aqueous products is due the removal of ash constituents which are dissolved into the aqueous co-product. This ash reduction also contributes to increased energy content of the hydrochar, which results in higher energy densification of algal hydrochars relative to hydrochars produced from treatment of lignocellulosic feedstocks at comparable temperatures.

The aqueous co-products (ACP) from HTC of whole algae and LEA algae were also evaluated to identify high-value chemicals. Although there was a very large amount of non-volatile residue (NVR) in the aqueous phase from treatment of the algal materials as compared to treatment of lignocellulosic feedstocks, only a small fraction of the ACP was identified through the various methods used. Using three different methods to characterize ACP, approximately 1% of the starting dry mass was identified as high value chemicals from the treatment of Spirulina. The total organic carbon in the ACP accounts for less than half of the dissolved mass, but the elemental balance of the solids indicates that much of the unidentified dissolved solids are nitrogen-containing compounds. Results from earlier, studies with Scenedesmus Dimorphus showed that different amounts and types of sugars are produced from HTC treatment of a different strain of algae. Overall, higher concentrations of high-value chemicals were identified in the ACP from Scenedesmus. However, it should be noted that the two algae treated by HTC came from two different sources: the Spirulina was purchased from a health food supplier while the Scenedesmus was grown in local ponds. The different processing and handling histories of the two algae could contribute to the observed differences in their behaviors.

Despite the lipid extraction, the sugar-related products from HTC treatment of LEA and whole algae were quite similar. Energy densification of the hydrochars was also similar. However, a lower fraction of high-value chemicals was observed in the ACP from LEA, as compared to whole algae. Overall, the results of this Example 2 indicate that HTC can produce both an energy-dense hydrochar at much milder conditions than those required for lignocellulosic feedstocks, as well as a valuable aqueous product stream from whole and lipid-extracted algae. Relatively mild treatment temperatures were applied, and it is possible that additional high value chemicals could be produced as treatment temperatures are increased.

It is to be understood that the above discussion provides a detailed description of various embodiments. The above descriptions will enable those skilled in the art to make many departures from the particular examples described above to provide apparatuses constructed in accordance with the present disclosure. The embodiments are illustrative, and not intended to limit the scope of the present disclosure. The scope of the present disclosure is rather to be determined by the scope of the claims as issued and equivalents thereto. 

We claim:
 1. A method for producing solid fuel comprising: providing a feedstock comprising algae or delipidized algal residue and a liquid carrier; heating the feedstock at a temperature of between about 120° C. and about 250° C., and a pressure of between about 2 bar and about 40 bar for a period of between about 5 minutes and about 16 hours, thereby forming an amount of a solid hydrochar; collecting the hydrochar; and compressing the hydrochar into a compressed solid.
 2. The method of claim 1, wherein the feedstock comprises between about 2% and about 10% by weight algae or delipidized algal residue.
 3. The method of claim 1, wherein the feedstock comprises less than about 20% by weight of algae or delipidized algal residue.
 4. The method of claim 1, wherein the hydrochar is compressed at a temperature of between about 25° C. and about 200° C.
 5. The method of claim 1, wherein the hydrochar is compressed at a temperature of between about 100° C. and about 160° C.
 6. The method of claim 1, wherein the hydrochar is compressed at a temperature of between about 120° C. and about 180° C.
 7. The method of claim 1, wherein the hydrochar is compressed at a temperature of between about 140° C. and about 160° C.
 8. The method of claim 1, wherein the hydrochar is compressed at a temperature of less than about 200° C.
 9. The method of claim 1, wherein the hydrochar is compressed at a temperature less than a temperature typically used to compress hydrochar produced from the hydrothermal carbonization of lignocellulosic biomass.
 10. The method of claim 1, wherein the hydrochar is compressed without the addition of an external binder.
 11. The method of claim 1, wherein the hydrochar is capable of being compressed without the addition of an external binder.
 12. The method of claim 1, wherein the hydrochar is compressed essentially without the addition of an external binder.
 13. The method of claim 1, wherein the hydrochar is compressed with less than about 10% by weight of an external binder.
 14. The method of claim 1, wherein the hydrochar is compressed with less than about 5% by weight of an external binder.
 15. The method of claim 1, wherein the hydrochar is mixed with hydrochar produced from hydrothermal carbonization of a lignocellulosic feedstock and the mixture is compressed.
 16. The method of claim 1, wherein the temperature is between about 150° C. and about 250° C. and the pressure is between about 5 bar and about 40 bar, and the reaction time is between about 5 minutes and about 2 hours.
 17. The method of claim 1, wherein the temperature is between about 180° C. and about 250° C., the pressure is between about 5 bar and about 40 bar, and the reaction time is between about 5 minutes and about 1 hour.
 18. The method of claim 1, wherein the feedstock comprises delipidized algal residue.
 19. The method of claim 1, wherein the feedstock comprises whole algae.
 20. The method of claim 1, wherein the feedstock consists essentially of delipidized algal residue and/or algae and the liquid carrier. 